SOLUTION: Alfreida doubled her money in the stock market every 4 years. After 20 years, she had 120,000 dollars. How much did she start with?

Question 540376: Alfreida doubled her money in the stock market every 4 years. After 20 years, she had 120,000 dollars. How much did she start with?
Answer by jpg7n16(66) (Show Source): You can put this solution on YOUR website!
Since Alfreida doubled her money every 4 years, after 20 years she will have doubled her money 5 times.

So, we know she started with some amount of money $X, which doubled 5 times and became $120,000. That means her X was multiplied by 2 again and again - a total of 5 times. As an equation, that looks like this:

or (the much easier way) using exponents, the 2's are more easily read as:

and since 2^5 = 32, the formula then becomes

Then to solve for her original x, just divide each side by 32.

So we know Alfreida started with $3,750.
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